This invention relates generally to x-ray transmission and radionuclide emission imaging systems. In a primary application the invention relates to a diagnostic imaging system where dual-energy x-ray transmission data are used to derive material properties of the object imaged, which properties are then used directly to correct radionuclide emission data obtained simultaneously from the same object using the same imaging system.
Emission radionuclide imaging is a well-established technique for localization of lesions in diagnostic radiology and nuclear medicine. Briefly, a compound labelled with a radionuclide or a radionuclide itself is injected into a subject. The radiolabelled material concentrates in an organ or lesion of interest, or can show a concentration defect. At a prescribed time following injection, the pattern of concentration of the radiolabelled material is imaged by a rectilinear scanner, gamma camera, single photon emission computed tomography (SPECT) system or positron emission tomography (PET) system. The imaging procedure requires a means to define the vector path along which the emitted gamma-ray travels before striking the detector of the imaging system. In the first three systems a collimator (typically made of lead or other high-atomic number material is interposed between the object and the detector to define the gamma ray path; in PET the unique characteristics of positron annihilation radiation are coupled with electronic circuitry to define the vector path. In all cases, the only information obtained when a gamma-ray strikes the detector is the fact that the photon originated somewhere within the object along the vector path projected back from the detector. For projection imaging systems, a two-dimensional image is formed with the intensity of each picture element, or pixel, proportional to the number of photons striking the detector at that position. In SPECT and PET, the vector paths are determined for multiple projection positions, or views, of the object, and cross-sectional or tomographic images are reconstructed of the object using standard algorithms. Again, the intensity assigned to each vector path is proportional to the number of photons striking the detector originating along the path, and the intensity of each pixel in the reconstructed image is related to these vector path intensities obtained at multiple views.
Both for projection and CT radionuclide imaging it is desireable to obtain absolute values for radionuclide concentrations (or radionuclide uptake) at each point in the image. Attenuation of the emitted photons within the object, before they reach the detector, is a function of the energy of the photons and the exact composition of the material through which the photon vector path passes to reach the detector. Photons emitted deeper within the object have a higher probability of attenuation than photons emitted near the surface. In addition, the composition of the material (in terms of effective atomic number, Z, and electron density) affects the attenuation, with more attenuation if the path passes through high Z or high density regions. Thus, in order to calculate absolute uptake or concentration of a radionuclide in a region of an object it is required that the path length of each type of material or tissue (or effective Z and electron density path lengths) be known for each vector. Attenuation corrections for emitted photons are made from this knowledge, allowing accurate concentration values to be obtained.
Prior art in this field involves two approaches. The first approach uses conventional projection or CT images to estimate the material composition of the object, from which attenuation corrections can be estimated. This involves obtaining a conventional image, mapping the radionuclide distribution onto the conventional image, generating estimated material path lengths along the emission vector paths, and correcting the observed radionuclide distribution based on these path lengths. There are two fundamental limitations in this approach. First the conventional and radionuclide images are obtained sequentially using different instruments, generally in different rooms and often on different days. These factors lead to significant problems in misregistration of the conventional and radionuclide image data sets. While 3-D image matching algorithms have been applied to this problem, their inability to solve the problem has kept them from routine use. Only limited success is obtained in objects of relatively time-invariant composition such as the head, and the method is essentially useless in objects such as the chest or abdomen where motion is continuous. Secondly, conventional CT or projection images suffer from inaccuracies in determined material properties due to beam hardening and scattering among other effects. Therefore, the attenuation corrections applied to the radionuclide data for photons of often very different energy than the x-ray data are only estimates.
The second approach known in prior art is the use of a radionuclide transmission source to obtain total path length attenuation measurements, then to use these measurements to estimate attenuation corrections for the emitted photons from the administered radionuclide. Budinger and Gullberg, "Three-Dimensional Reconstruction in Nuclear Science," IEEE Transactions on Nuclear Science, June 1974, have suggested the use of multiple radionuclide photon energies to obtain material-selective path length data, and Peppler, "Combined Transmission-Emission Scanning Using Dual-Photon Absorptiometry,"Ph. D. Thesis, University of Wisconsin, 1981, has used sequential measurements, using two radionuclide energies to obtain data about bone and soft-tissue path lengths followed by acquisition of emission data which was then corrected by the transmission measurements. Some of these prior approaches use simultaneous or near simultaneous acquisition of transmission and emission data while others use sequential acquisition. However, all have the fundamental limitation that the use of a radionuclide transmission source to determine effective material path lengths limits the accuracy of these measurements due to statistical noise in the transmitted photon intensity measurements.
Material-selective imaging using x-ray sources has been described by Alvarez and Macovski in U.S. Pat. No. 4,029,963. This method or modification thereof can be used to determine effective material path lengths using two sequentially or simultaneously-acquired data sets at two different effective photon energies. Once the object being imaged is decomposed into a basis set of two properties, a priori knowledge of the energy dependence of the basis properties can be used to reconstruct an image of the object at the exact energy of the radionuclide emission photon being imaged. From this image is derived exact attenuation corrections for the emitted photons.
The present invention overcomes the fundamental limitations of the prior art. Specifically, transmitted photons from an x-ray source are acquired simultaneously with emitted photons from the contained radionuclide source, using the same photon detector in an identical geometry, so all the problems of misregistration are overcome. The dual-energy x-ray projection data are solved exactly for material-specific properties and recombined into an effectively monoenergetic image, eliminating inaccuracies in material property estimation due to beam hardening. The high photon flux from the x-ray tube overcomes the accuracy limitation of radionuclide transmission sources.